The present invention relates to a method and a device for the production of curved spring strip sections, specifically having a variable material thickness over their trimmed length.
A method and a device of the kind described above are known from WO 99/51375. In the known method, a spring strip composed of the spring strip sections disposed sequentially to form one piece is bent between three support points spaced apart in the direction of spring strip travel and impinging alternately in sequence on one of the two sides of the strip and then reverse-bent in the opposite direction in a subsequent fourth support point by a lesser degree of curvature. The spring strip section so treated is then detached from the spring strip. The second of the three support points for bending the spring strip and the fourth support point for reverse bending the spring strip are both configured to be adjustable perpendicular to the spring strip in the direction of strip thickness. The perpendicular adjustments of the adjustable support points are controlled relative to the spring strip in accordance with predetermined programs, which take account of the changing material thickness within the spring strip sections in the direction of spring strip travel. A spring strip section produced with the help of this method is characterized by high dimensional stability. Furthermore, the method permits a continuous bending process in a single step, corresponding to one pass through the support points.
The known device has a bending unit consisting of three spaced apart support points, through which a spring strip composed of the spring strip sections disposed behind one another to form one piece can be passed in such a way that the three support points impinge on the one and the other side of the strip sequentially and alternately in the direction of spring strip travel. The second of the three support points is configured to be adjustable perpendicular to the spring strip to set a radius of curvature. Furthermore, the known device has a reverse bending unit positioned after the bending unit in the direction of spring strip travel. The reverse bending unit comprises a fourth support point acting upon the same side of the spring strip as the second of the three support points. This fourth support point is adjustable perpendicular to the spring strip to set a reverse radius of curvature. Finally, the known device possesses a cutting unit to detach the spring strip section from the spring strip after it has passed through the bending unit and the reverse bending unit.
Bent spring strip sections which are produced by means of such methods and devices are used, for example, in beam blade windshield wipers, which are known from U.S. Pat. No. 3,192,551. With a beam blade windshield wiper of this type, the spring strip section, on whose center the particular windshield wiper arm acts, exerts almost even downward force on the wiping element, which is secured at the back, against the generally curved surface of the windshield over the entire wiped area. The thickness of the material of the spring strip section employed therein changes over its length, being at its maximum in the middle of the spine and decreasing toward the two ends of the spine.
To produce beam blade windshield wipers of this type, it is known from WO 99/52753 to conjoin the curved spring strip section to a flexible rubber wiping element and to attach a connecting device for a wiper arm.
The known procedure, or the known device respectively, for producing curved spring strip sections demands a relatively complex construction and relatively great expense if the lever arms operant in the bending process and/or in the reverse bending process are to be altered. These disadvantages are to be overcome with the present invention.
From DE-PS 458 563 it is known to process a spring strip composed of spring strip sections disposed sequentially to form one piece in a machining station with a punch and stamping tool. After processing, the spring strip sections so treated are detached from the spring strip.
From WO 94/17932 both a method and a device are known to produce spring strip sections having varying material thickness. A spring strip of constant thickness and width is pulled through a pair of oppositely located rollers, where the spacing of the rollers is varied by a control device to form the particular spring strip sections with a varying thickness along their length.
From DE-OS 38 15 304 a method and a device are known for bending or straightening profile steel and strips using cylinders or rollers. Here too, the support points, namely the cylinders or rollers, are staggered alternately on both sides of the strip to be shaped, where a staggered arrangement is understood to mean that the support points are positioned offset to one another, alternately, and in sequence on the one or the other side of the strip. All the support points are adjustable individually perpendicular to the strip in the direction of strip thickness. It has been shown that the strips produced in this way have far less, if any, residual stresses. Continuous bending operation can also be implemented.
In DE-OS 38 15 304 already mentioned, a straightening process is described in addition to a bending process, where straightening differs from bending in that straightening consists to a certain degree in a double application of the bending process described, wherein the second bending is performed in counter-phase to the first bending, so that bending from the particular plane of reference in one direction is followed by bending from the plane of reference in the other direction by additional support points. Accordingly, the support points in straightening are also adjusted perpendicular to the strip in the direction of strip thickness to achieve counter-phased deflections.
The device for straightening known from the above described DE-OS 38 15 304 differs from the one for bending in that the support points are arranged in such a way that straightening can take place by bending two times in opposite directions to the reference plane, so that in principle the device can be used for both bending and straightening. Depending on the required accuracy of the radii of curvature, reverse bends are made.
It is pointed out in this document (DE-OS 38 15 304) that straightening is generally performed in practice by cold-forming in order to bring bent or distorted work pieces to the desire geometric shape, but there are no indications to the effect that a desired geometric shape could mean a shape deviating from a non-curved or unbent shape. Straightening is generally understood to mean a bending process in which the straightened end product has a quasi-infinite radius of curvature and thus possesses maximum straightness, or minimal residual curvature. Accordingly, straightening is a subform of bending with successive, opposite deformations with decreasing radius of curvature (see for example, Thomas Neff, “Roller Straightening Machines—Typology and Characteristic Features,” in “Strips, Sheets, Pipes” 6–1976, pp 223–226). The same can be found in the Handbook of Manufacturing Equipment, Vol 2/3 “Forming and Shearing,” page 1288, Carl Hanser Verlag, Munich, Vienna, 1985, where roller straightening is presented as a subform of roller bending.
To be sure it is basically known from the above-cited essay by Thomas Neff that roller-straightening machines with restricted parallel straightening space can be used to create predetermined curvatures, i.e., as bending machines (see page 225, section 3.1.1) so that a curved shape can be produced by bending and subsequent lesser reverse bending, but there is no express indication that this can be achieved with individually adjustable support points.
A straightening device is known from DE 195 03 850 C1 in which individual straightening rollers or straightening rollers assembled in groups are adjustable to achieve a desired degree of straightening, where the adjustment is made depending on a downstream analysis device. The intention here is to straighten strip material; the production of bent strip sections is not explained.
From the dissertation for the Faculty of Mechanical Engineering at the University of Karlsruhe by Werner Klein from 1979, entitled “Mechanical and Radiographic Studies on the Characteristics of Unalloyed Steels at Room Temperature,” it is known that with reverse bending deformation the inhomogeneous condition of residual stress is alleviated after even minor reverse deformation (see page 97). This dissertation also deals with bending deformation with subsequent minor reverse deformation which is performed in conjunction with straightening. In the case of the present invention, however, curved spring steel sections are to be produced with a desired, geometric shape and not straightened spring steel sections.